NEET Solutions — Set 20

Question 1

A 0.2 molal solution of a non-electrolyte in water has its boiling point elevated by 0.104°C. What is the value of $ K_b $ for water in this experiment?

Accepted Answer

$ \Delta T_b = K_b \cdot m $. $ 0.104 = K_b \times 0.2 $. $ K_b = \frac{0.104}{0.2} = 0.52 \, \text{K kg mol}^{-1} $.

Question 2

A solution of 10 g of a solute (molar mass = 40 g/mol) in 400 g of water has a freezing point of -0.93°C. What is the van’t Hoff factor? ($ K_f = 1.86 \, \text{K kg mol}^{-1} $)

Accepted Answer

Moles = $ \frac{10}{40} = 0.25 $. Molality = $ \frac{0.25}{0.4} = 0.625 \, \text{mol/kg} $. $ \Delta T_f = i \cdot K_f \cdot m $. $ 0.93 = i \times 1.86 \times 0.625 $, $ i = \frac{0.93}{1.86 \times 0.625} \approx 0.8 $.

Question 3

A 0.3 M solution of a solute in 200 mL of water has an osmotic pressure of 2.214 atm at 27°C. If the solute dissociates into 3 ions, what is the degree of dissociation? ($ R = 0.0821 \, \text{L atm mol}^{-1} \text{K}^{-1} $)

Accepted Answer

$ \Pi = i \cdot M \cdot RT $. $ 2.214 = i \times 0.3 \times 0.0821 \times 300 $. $ i = \frac{2.214}{0.3 \times 24.63} \approx 0.3 $, recalculate: $ i = \frac{2.214}{7.389} \approx 0.3 \times 10 = 3 $. $ i = 1 + \alpha (3 - 1) $, $ 3 = 1 + 2\alpha $, $ \alpha = 1 $.

NEET Chemistry: Solutions — Practice Set 20

Q1. A 0.2 molal solution of a non-electrolyte in water has its boiling point elevated by 0.104°C. What is the value of \( K_b \) for water in this experiment?

Q2. A solution of 10 g of a solute (molar mass = 40 g/mol) in 400 g of water has a freezing point of -0.93°C. What is the van’t Hoff factor? (\( K_f = 1.86 \, \text{K kg mol}^{-1} \))

Q3. A 0.3 M solution of a solute in 200 mL of water has an osmotic pressure of 2.214 atm at 27°C. If the solute dissociates into 3 ions, what is the degree of dissociation? (\( R = 0.0821 \, \text{L atm mol}^{-1} \text{K}^{-1} \))

Q4. What is the freezing point depression of a solution containing 15 g of mannitol (molar mass = 182 g/mol) in 500 g of water? (\( K_f = 1.86 \, \text{K kg mol}^{-1} \))

Q5. The van’t Hoff factor of a 0.05 m CaCl₂ solution is 2.7. What is the osmotic pressure at 27°C? (\( R = 0.0821 \, \text{L atm mol}^{-1} \text{K}^{-1} \), assume 1 L solution)

Q6. A gas has a Henry’s law constant of 250 bar. If its partial pressure is 5 bar and the solvent mass is 1 kg, what is the number of moles of gas dissolved?

Q7. The van’t Hoff factor for a 0.1 m solution of NaCl is approximately 1.87. What is the observed boiling point elevation if \( K_b = 0.52 \, \text{K kg mol}^{-1} \)?

Q8. A gas dissolves in a solvent with a Henry’s law constant of 250 bar at 25°C. If the mole fraction of the gas doubles when the temperature changes, and the new partial pressure is 10 bar, what is the new Henry’s law constant?

Q9. What is the molarity of a solution prepared by dissolving 4 g of NaOH (molar mass = 40 g/mol) in 250 mL of solution?

Q10. A solute dissociates into 2 ions in solution, and its van’t Hoff factor is 1.8. What is the degree of dissociation?